Please run:
sudo apt-get update -o Debug::pkgAcquire::Worker=1 2>&1 | tee /tmp/apt-1236258-update-with-debug.log
and attach the resulting /tmp/apt-1236258-update-with-debug.log to the bugreport.
Mentioning the exact sources.list line you are using to configure the mirror would help to.

Its a rather obscure error, I at least haven't seen it so far in my few years around apt, so mention any APT-configuration you have changed. Also, check that the packages 'apt', 'libapt-pkg4.12' and 'libapt-inst1.5' have the same version.

Oh, and don't install debian packages for key software like APT, in this very case its the same team and not that many ubuntu-specific patches applied (which aren't critical), but its not a good idea in general.

I promise I will not install debian packages on ubuntu anymore ;) But I did not play with these packages before the problems occured, I installed the saucy ubuntu packages again. i did not modify my apt config, I had some external repos in /etc/apt/sources.lists.d but these are all disabled now.